We know how to manufacture resistance chips in such a way as to form a resisting element or resistive layer applied on an electrically insulated substratum in a square or rectangular shape of a few square millimeters.
The laying of this resisting element is realized by silkscreen printing with pastes or resisting inks layed directly on this substratum. The thickness of the layer applied is in the order of several micrometers and its electrical resistance varies between few ohms and several megaohms. This technique is known by people in the field under the specific term of deposit in thick layers. We also know how to manufacture the same type of components by layering by vacuum depositing technique of resisting materials notably of the chromium-nickel type or Constantan directly on the said substratum. Under these conditions, the ohmic value of the component so realized may vary between few ohms and few tens of kiloohms, the thickness of the layer varying typically between 10 and few thousand nanometers. This technique is known under the specific term of vacuum depositing. The extremity electrodes of these known resistances are made according to techniques of layering by thick layers, notably by deposit of Ag-Pd alloys on the substratum, done in such a way as to form an electrical continuum with the resisting material and by recharging later by electrolytic techniques the said Ag-Pd alloy with thick nickel, Sn and Pd-Sn layers.
The fabrication of these resistances in chips according to the depositing in thick or thin layers is realized by forming the resisting layer on a large insulating substratum, in the order of few tens of square centimeters and by dividing later the substratum by sections in comb or strip shapes. The resisting element or resistance layer is protected by a protective layer of organic matter of the photoresist type. The extremity electrodes are formed on the top of the component and the whole is treated at high temperature in order to give to the said electrodes an as weak as possible conductibility as well as a good mechanical hold.
Each of the sections in strip shape is then cut in units of a few square millimeters and finally an electrolytic deposit of Ni and Pb-Sn or equivalent is applied on each chip. This way, we obtain a resistance in the form of a surface mounted chip.
This process is described for example in the DE-A-3 148 778, the U.S. Pat. No. 4,278,706, the EP-A-0 191 538 and the U.S. Pat. No. 4,792,781.
The resistances manufactured by these known processes present however the disadvantage, by their nature, not to be precise and to have characteristical temperature and response variations in frequency prejudicial to the performances expected today for electronic circuits.
Indeed, the tolerances in ohmic value of these resistances are seldom lower than few per cent of the nominal value of the resistance. Also, their temperature coefficient, represented by the variation of the nominal resistance according to the temperature is never lower than 100 to 200 parts per millions/degree Celsius (ppm/.degree.C.).
Moreover, the variations of the nominal resistance with time, can be between few thousands and serveral thousands parts per million (ppm).